In systems for direct current telegraphy, more particularly in the repeaters of such systems, the formerly-used mechanical telegraph relays are now replaced to an increasing extent by electronic telegraph relays which offer important advantages as regards life, space occupied and price. For use of such telegraph relays in single-current telegraphy, the electronic equivalent of a mechanical single pole relay must be provided. Double-current telegraphy on the contrary requires a double pole relay.
An electronic relay arrangement of the type described in the preceding paragraph is known from Swiss Pat. No. 567,350. This arrangement is only suitable for single-current telegraphy systems and comprises a voltage regulator circuit in series with the telegraph line and across which circuit a light-emitting diode is connected and further comprises a voltage limiter circuit connected across the telegraph lines. Below a lower line current level no current is permitted to flow through the LED while between that lower level and an upper level the current through the LED linearly relates to the line current. Above the upper level the LED-current is held at the upper level by the voltage limiter circuit. This arrangement has the drawback that the maximum current through the LED is high which result in faster aging of this component. Further, the phototransistor which is used to collect the emitted radiation of the LED calls for an operating range which is at least as wide as that of the LED.
It is an object of the invention to provide an electronic relay arrangement of the type mentioned in the opening paragraph which obviates the above-mentioned drawbacks and provides, by means of a simple circuit, a reliable electronic relay arrangement which is little sensitive to interference. According to the invention an electronic relay arrangement of the type defined in the opening paragraph is characterized in that the current switching device is in a first of exclusively two states when the line current value is below the threshold current value and in the second (responding) state of the current switching device, a control signal is applied to the second terminal for causing a current to flow through the transmission circuit with a predetermined constant value.
The electronic relay arrangement according to the invention has the advantage that the current switching device is fed via the telegraph line. A further advantage is that the predetermined constant current through the transmission circuit supplies a defined supply current for a transmitting element (for example a light-emitting diode or a transformer winding) in the transmission circuit and that the value of this current can be chosen so (low) that a long life of these elements is ensured. A still further advantage is that the current switching device responds to the absolute value of the line current so that the current switching device is polarity insensitive and, consequently, not only suitable for single-current telegraphy but also for double-current telegraphy.
It is a further object of the invention to provide a current switching device comprising a small number of components, which components need only have a low degree of precision and by means of which an accurate and reliable current detection and current switch-over can nevertheless be effected. A current switching device for use in an electronic relay arrangement according to the invention is characterized in that the current switching device comprises a first, a second and a third transistor, in that the emitter of the first transistor is connected to the emitter of the second transistor, to the base of the third transmitter and, via a first resistor, to the output terminal in that the collector of the first transistor is connected to the first terminal, to the base of the second transistor via a first diode--which is poled into the direction of the base-emitter diode of the second transistor--and via a second resistor to the base of the first transistor, in that the collector of the second transistor is connected to the second terminal and the base of the second transistor via a third resistor to the output terminal, and in that the emitter of the third transistor is connected to the output terminal and the collector of the third transistor to the base of the first transistor.
An electronic relay arrangement for double-current telegraphy is characterized according to a further embodiment of the invention in that the current switching device is included in a diode-bridge rectifier circuit. This has the advantage that the threshold value is identical for both polarities of the line current without the need for precision components.
A further embodiment of an electronic relay arrangement for single-current telegraphy according to the invention is characterized in that the transmission circuit comprises a first terminal coupled to the first connecting terminal of the line for deriving the above-mentioned current having the predetermined constant value from the current flowing in the line, and that said current flows from the first connecting terminal, through the transmission circuit and the control input to the second connecting terminal. This measure has the advantage that the current flowing in the telegraph line can be utilized for feeding the transmission circuit.
A further embodiment of an electronic relay arrangement for double-current telegraphy according to the invention is characterized in that the transmission circuit comprises a second terminal coupled to the second connecting terminal of the line for deriving the above-mentioned current having the predetermined constant value from the current flowing in the line, the polarity of which is opposite to the polarity of the current through the first input terminal and that said current flows from the second connecting terminal through the transmission circuit and the control input to the first connecting terminal. An advantage thereof is that the polarity of the line current is available in the transmission circuit and that it can therefore be transmitted. A further advantage is that the control input of the transmission network can be used in common for both polarities.